1. Field of the Invention
The present invention relates to an image taking apparatus suitable for, for example, a digital still camera, a video camera, a TV camera and a monitoring camera.
2. Description of the Related Art
As an image taking optical system to be used in an image taking apparatus, a compact high-performance zoom lens which is short in total lens length, and has a wide angle of field is required. As a zoom lens which is compact in its entirety and has a wide angle of field, there is known a negative-lead type zoom lens in which a lens unit having a negative refractive power is disposed on the most object side.
As the negative-lead type zoom lens, there is known a three-lens-unit zoom lens including, in order from the object side to the image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a positive refractive power. For example, there is known a compact zoom lens having a wide angle of field, in which the first lens unit includes a negative lens and a positive lens, the second lens unit includes a positive lens, a cemented lens in which a positive lens and a negative lens are cemented and a positive lens, and the third lens unit includes a positive lens. (U.S. Pat. Nos. 7,092,170, 7,023,623, and Japanese Patent Application Laid-Open No. 2006-208890)
For the purpose of downsizing the zoom lens, there is known a retractable type in which, when the zoom lens is not in use (at the time of non-image taking), the intervals among the respective lens units are reduced to be different from the intervals used in an image taking state so as to reduce the amount of lens protrusion from the image taking apparatus. In general, as the number of lenses of each lens unit constituting the zoom lens becomes larger, the length of each lens unit along the optical axis becomes longer. In addition, as the amount of movement of each lens unit at the time of zooming and focusing becomes larger, the total lens length becomes longer. As a result, the length of a retractable lens in retracted state becomes long, and thus it is difficult to downsize the image taking apparatus.
In order to obtain a zoom lens which is compact in its entirety and short in length of the retractable lens in retracted state, and which has a wide angle of field and a high zoom ratio, it is necessary to appropriately set the movement condition of each lens unit, which depends on the zoom type and zooming, the refractive power of each lens unit and the lens structure of each lens unit. For example, when the zoom lens is desired to be downsized, this is achieved by strengthening the refractive power of each lens unit and reducing the movement amount of each lens unit during zooming.
However, simply strengthening the refractive power of each lens unit causes larger aberration fluctuations due to zooming, and those are difficult to be corrected satisfactorily. When the above-mentioned three-lens-unit zoom lens is desired to be made compact in its entirety and to widen angle of field, it is important to appropriately set the structure of each lens of the second lens unit serving as a main magnification-varying lens unit. If the lens structure of the second lens unit is set inappropriately, it becomes difficult to obtain a wide angle of field and high optical performance over the entire zoom range while achieving the downsizing of the entire system.